Process for making current limiting devices



Oct. 13, 11970 c, J, MORATls 3,533,966

PROCESS FOR MAKING CURRENT LIMITING DEVICES Filed Feb. 11, 1966 3 Sheets-Sheet 1 350F/HR.

75F/HR.

500F/HR.

Fig.3

INVENTOR Chrisfy J. Moruris A TORNEY Oct. 13, 1970 I,

PROCESS FOR MAKING CURRENT LIMITING DEVICES.

Filed Feb. 11. 1966 VOLTAGE (AC) (ms) SELF-HEATING POINT c. J. MORATIS 3,533,966

3 Sheets-Sheet 2 500F/HR.

350F/HR.

THERMAL RUNAWAY REGION I- MA (AC) FIG.4.

PROCESS FOR MAKING CURRENT LIMITING DEVICES Filed Feb. 11, 1966 C. J. MORATIS Oct. 13, 1970 5 Sheets-Shed I5 O O 4 3 2 w QEE H O I TIME IN SECONDS FFIGB.

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I00 I TEMPERATURE --C United States Patent 3,533,966 PROCESS FOR MAKING CURRENT LIMITING DEVICES Christy J. Moratis, Penn Hills, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 11, 1966, Ser. No. 526,806 Int. Cl. H01b; H01c U.S. Cl. 252520 7 Claims ABSTRACT OF THE DISCLOSURE A process is described for the treatment of a positive temperature coetficient of resistance thermistor of the doped barium titanate type. The process, which includes a critical heating step, is effective for permitting the thermistor to be used at voltages in excess of 50 to 100 volts.

This invention relates to a process for making current limiting devices or thermistors comprising ceramic bodies having a high positive temperature coetficient of electrical resistance and a relatively high voltage rating,

Certain ceramic semiconductor materials possess negative temperature coefficients of resistance, that is, the electrical resistance decreases as the temperature increases, while other ceramic semiconductor materials possess positive temperature coefficients of resistance. In materials of the latter type, the electrical resistance increases as the temperature increases. Materials of this latter type in which the electrical resistance increases abruptly, so that in the range of a few degrees the electrical resistance increases many times to a high value, are disclosed in U.S. Pat. No. 2,981,699, dated Apr. 25, 1961 and U.S. Pat. No. 2,976,505, dated Mar. 21, 1961.

Ceramic semiconductor materials of the positive temperature coefficient type exhibit the described increase in resistance at the Curie temperature (self-heating point). The materials of these patents are composed primarily of barium titanate which has a Curie temperature of approximately 110 C. The abrupt change in resistance occurs when the semiconductor material undergoes a phase change in its crystalline structure. The Curie temperature of the basic barium titanate material can be adjusted to some extent by additives, since it is decreased by additions of strontium and increased by additions of lead. Small additions of one or more rare-earth elements such as yttrium, cerium, neodymium and Samarium are utilized to reduce the resistivity of these materials.

Current limiting devices have been made from the materials disclosed in the patents mentioned above in which, after the initial flow of current heats the device to its Curie or switching temperature, the resistance of the material rises to such a point as to attenuate the current to a very low value. The devices which have been made, however, have been limited to operations in circuits having relatively low voltages, for example, 50 to 100 volts or less. Devices capable of operating at voltages substantially in excess of 50 to 100 volts would, of course, have a wider range of application.

The object of the present invention is to provide an improved process for making semiconductor ceramic compositions having a positive temperature coefficient and capable of operating at voltages substantially in excess of 50 to 100 volts.

Another object of the invention is to prepare semiconductor ceramic compositions characterized by a high positive temperature coefficient having a relatively fine grain size whereby the materials can withstand relatively high voltages substantially in excess of 50 to 100' volts.

3,533,966 Patented Oct. 13,, 1970 A still further object of the invention is to provide in a process for making high positive temperature coefficient ceramic materials, a controlled heating to the sintering temperature whereby a fine grain size is produced which makes the material capable of withstanding relatively high electrical potentials substantially in excess of 50 to volts.

Other objects of the invention will in part, be obvious and will, in part, appear hereinafter.

For a better understanding of the nature and scope of the invention, reference should be had to the following drawings, in which:

FIGS. 1-3 comprise three photomicrographs illustrating the microstructure of samples in which these different heating rates to the sintering temperature have been used;

FIG. 4 is a graph in which voltage is plotted against current for three samples of a ceramic semiconductor material having a positive temperature coefiicient of electrical resistance and which have been subjected to different rates of heating to the sintering temperature;

FIG. 5 is a pair of graphs in which voltage and current are plotted against time for a material treated in accordance with the present invention;

FIG. 6 is a pair of curves as taken from a recording device showing the current attenuation at constant voltage in an actual test of a device made in accordance with this invention; and

FIG. 7 is a graph showing the effect of increasing amounts of strontium oxide or lead oxide on the Curie temperature of barium titanate.

In accordance with the present invention it has been found that by employing a rapid rate of heating to the sintering temperature, a fine grain microstructure can be obtained which greatly increases the voltage rating of the semiconductor ceramic materials described. To achieve this desirable result, the rate of heating to the sintering temperature must be at least 350 F. per hour and preferably in the range of 500 F. to 600 F. per hour.

The process of the invention is applicable to the following ceramic semiconductor materials all of which are characterized by a positive temperature coelficient of electrical resistance:

Samarium and neodymium may replace a part or all of the yttrium and cerium in the above formulations.

The process of the invention involves calcining a mixture of the raw ingredients, which may be the oxides, carbonates or nitrates thereof to decompose them in part and to partially react the 'solid components, wet milling in a ball mill the calcined components to obtain a fine particle size to improve the reactivity thereof, drying the milled components and adding a suitabe temporary.

binder for example, wax or polyvinyl alcohol thereto so that a compact or pellet can be formed by pressing, placing the pellet in a furnace and, in a two stage sintering operation, first heating the pellet at a rate not less than 350 F. per hour in a protective atmosphere such as argon, nitrogen or helium to a temperature in the range 3 from 2000 F. to 2500 F. for a period of from /2 to 4 hours and, second, subjecting the pellet to an oxidizing atmosphere at a temperature of from 1830 F. to 2470 F. for from minutes to 1 hour.

The sintering operation may alternatively be carried out in a single stage under a controlled atmosphere of argon-oxygen or nitrogen-oxygen in the range of 60 to 90 parts of argon or nitrogen and the balance 40 to 10 parts of oxygen. This procedure provides closer process control than the two stage sintering operation. In this case sintering would be carried out for a period of from /2 to 4 hours at a temperature of from 2000 F. to 2500 F.

It will be understood that the sintering time depends in part on the mass of the pellet, shorter sintering times being applicable to smaller pellets.

The following example illustrates the process of the invention in making a current limiting device having a switching temperature of about 176 F.

EXAMPLE A material of the composition Y Ba Sr TiO was prepared in the mol proportions indicated from the following commercially available C.P. quality compounds: yttrium oxide (Y O barium carbonate (BaCo titanium dioxide (TiO strontium carbonate (SrCo The yttrium oxide is added as the nitrate formed by dissolving the oxide in nitric acid. The ingredients were weighed, then ball milled in water in a porcelain jar mill for 6 hours. The milled material was dried and then calcined at 1900 F. for 2 hours to decompose the nitrates and carbonates and to partially react the components. After calcination the material was wet milled for 16 hours. The milled material was then dried and a wax binder was added to impart adequate strength to the sample. The milled material was then pressed at approximately 10,000 p.s.i. to form a 0.375 inch diameter by 0.10 inch high disc.

The ceramic discs were sintered at approximately 2350 F. in argon for about A; hour and then subjected to an oxygen atmosphere at 2200 F. for about 10 or 12 minutes. The rate of heating to the sintering temperature was varied, heating rates of 75 F. per hour, 350 F. per hour and 500 F. per hour were employed. The microstructure obtained with each rate of heating is shown in FIGS. 1 to 3. It is clear that the sample heated to the sintering temperature at the rate of 500 F. per hour has an extremely fine grain size.

Electrodes were applied to the sintered ceramic discs by flame spraying an indium alloy powder onto the circular faces. Electrical data were obtained for both static and dynamic conditions. The static volt-ampere (V-I) data were obtained by recording the steady state current at various voltage levels for a constant environment. These data gave the self-heating point, current limiting region and the thermal runaway region. Each of the three curves of FIG. 4 represents a characteristic curve obtained in testing material which had been heated to the sintering temperature at one of the three rates mentioned above. It is apparent that the higher heating rates are beneficial in that devices having higher voltage ratings may be made from materials so treated. The device is rated at some voltage below the level of the thermal runaway region.

Dynamic data were obtained by applying a fixed voltage to the ceramic discs and measuring the current attenuation with time under constant environmental conditions. Characteristic voltage and current curves are set forth in FIG. 5. The most significant feature of these curves is the abrupt and rapid attenuation of the current at constant voltage.

An examination of the photomicrographs of FIGS. 1 to 3 indicate that the higher heating rates result in materials with smaller grain size, which is beneficial from the standpoint of voltage rating. The material as seen under the microscope can be described as consisting of two electrically active portions. One portion is the grain body itself which is a relatively low resistivity region, and the second portion, which is the grain boundary region, has a relatively high resistivity. It appears "that the resistancetemperature anomaly found in materials of this type is related to the degree of oxidation of the grain boundaries. It will thus be seen that if the voltage stress can be distributed over a larger number of grain boundaries per unit length, then the voltage gradient may be increased proportionally. A larger number of grain boundaries per unit length may be obtained by appropriate processing to reduce the grain size. Grain growth may be restricted by rapid heating to the sintering temperature and/or by the addition of grain growth inhibitors such as A1 0 It must be understood that the small grain size material has a higher resistivity and this must be taken into consideration, since in such a material the amount of current which can be passed is reduced.

Other compositions have been treated in accordance with this invention with very favorable results. Thus, pellets having the composition Y Ba TiO made as described above, are rated at volts when heated to the sintering temperature at the rate of 75 F. per hour. When the rate employed is increased to 350 F. or 500 F. per hour, the device is rated at volts.

Pellets having the composition have also been made. The A1 0 is added as fine particles which will pass through a 400 mesh screen. The A1 0 functions as a grain growth inhibitor. These pellets were rated at 70 volts when heated to the sintering temperature at the rate of 75 F. per hour. When the rate of heating was increased to 350 F. per hour, the voltage rating increased to 150 volts and at a rate of 500 F. per hour the voltage rating was volts. Pellets having the nominal composition Y Pb Ba TiO have been made and have also exhibited increased voltage rating at the higher rates of heating of 350 F./hour and 500 F./hour.

Devices of the type which have been described may be used in place of a motor start-winding switch where a large flow of current is desired for a brief interval of time. The device is placed in series with the start-winding of a split-phase motor. When voltage is applied directly to the current limiting device start-winding combination, a surge of current will flow that will energize the start-winding. The initial flow of current will also self-heat the current limiting device and switch it into its high resistance state in which the current flow will be limited to some extremely small value. The greater portion of the voltage input will appear across the limiter device after the motor has started. In order for the limiter device to return to its initial low resistance state, the voltage source must be interrupted and the device permitted to cool below its switching temperature. The current limiter device start-winding combination is described in detail in the copending application of Elder and Dow, Ser. No. 520,475, filed Jan. 13, 1966, now abandoned.

In one device made in accordance with this invention in which a 500 F. per hour rate of heating to the sintering temperature is employed, a 200 volt input produces a 7.7 ampere inrush that is attenuated to approximately 0.06 ampere in lesss than one second. The voltage and current curves for this device as recorded by a two channel Brush (Mark H) recorder are set forth in FIG. 6. The power dissipation at time zero is approximately 1.5 kw. (15 kw. per square inch) for the 0.375 inch diameter device.

The Curie temperature of barium titanate may be raised or lowered by appropriate additions of PhD and SrO, respectively. The effect of these additives on the Curie temperature is set forth in FIG. 7. Lead oxide increases the Curie temperature while strontium oxide decreases the Curie temperature.

The response time, the time required to self-heat to a high resistance level, in the devices of this invention can vary from 0.1 second to several seconds depending on the applied voltage, current flow, heat sink, and the thermal characteristics of the encapsulation structure and materials.

What is claimed is:

1. In a process for making a ceramic body having a positive temperature coefficient and a voltage rating substantially in excess of 50 to 100 volts with a composition corresponding to one of the formulas,

Y Ba TiO --where x=.005 to .02

Ce Ba TiO where x=.003 to .006

Y Ba Sr TiO where m has a value of from 0.005

to 0.02 and x' 0.01 to 0.30

Y Ba Pb TiO -where n=0.005 to 0.02 and x:

Ce Ba Pb TiO -where n=0.005 to 0.02 and x:

Ce Ba Sr TiO -where n=0.003 to 0.006 and x:

including the steps of calcining a mixture of raw ingredients to decompose them in part and to partially react the solid components thereof, milling the calcined components to obtain a fine particle size, adding a suitable binder to the milled components forming a predetermined shape and sintering the predetermined shape at a temperature within the range between 1830 F. and 2500 F., the improvement comprising heating the predetermined shape to the sintering temperature at a rate not less than 350 F./ hour.

2. The process of claim 1 wherein the heating is carried out in an atmosphere capable of oxidizing the pellet to produce the desired formula, for a period of from /2 to 4 hours.

3. The process of claim 2 wherein the atmosphere is selected from the group consisting of (1) 60 to 90 parts of argon and the balance 40 to parts of oxygen and (2) 60 to 90 parts of nitrogen and the balance 40 to 10 parts of oxygen.

4. The process of claim 1 wherein the predetermined shape is heated to the sintering temperature at a rate of from 500 F. to 600 F. per hour.

5. The process of claim 3 wherein the predetermined shape is heated to the sintering temperature at a rate of from 500 F. to 600 F. per hour.

6. In the process of producing doped barium titanate positive temperature coefiicient thermistors of predetermined composition wherein the composition of the formula is produced by calcining a homogeneous mixtureof (1) titanium oxide, (2) at least one material from the group consisting of the oxides and carbonates of barium, strontium and lead, and (3) a small but critical amount of yttrium nitrate, milling the resulting product, compac ting it to a pellet of desired shape and then heating the compacted pellet to a sintering temperature within the range between 2000 -F. and 2500 F., the improvement comprising heating the pellet to the sintering temperature at the rate of 500 to 600 F./hour.

7. The process of claim 6 wherein the atmosphere is selected from the group consisting of (1) to parts of argon and the balance 40 to 10 parts of oxygen and (2) 60 to 90 parts of nitrogen and the balance 40 to 10 parts of oxygen.

References Cited UNITED STATES PATENTS 2,976,505 3/1961 Ichikawa 25252O 2,981,699 4/1961 Ichikawa 252520 3,020,619 2/1962 Koch 264-66 3,176,056 3/1965 Hicks 264-66 3,277,222 10/1966 Vachet et al. 26466 3,341,473 9/1967 Loch 252-520 3,377,561 3/1968 Sauer 252520 J. D. WELSH, Primary Examiner US. Cl. X.R. 

